In several of its more specific embodiments and aspects, the present invention is related to mobile cellular radio systems, and more particularly to microcellular radio systems (intended for slow-moving, rather than vehicular, users). One of the objects of the present invention, in one or more of its particular aspects, is to provide a mechanism by which a microcellular radio system may utilize radio channels which are also assigned to fixed radio systems, such as point-to-point systems (e.g., Private Operational Fixed Microwave--POFM).
While several countries, including the United Kingdom, and other countries in Europe and Asia, have each reserved an exclusive spectrum for implementation of the next generation of wireless Personal Communications Services (PCS), the United States has not reserved such a clear and unassigned spectrum for PCS. In the United States, incumbent radio systems such as POFM have already been assigned use of certain portions of the frequency spectrum (between 1850 and 2200 MHz), and accordingly restrain the ability to implement and thus utilize emerging PCS technologies.
New techniques have been proposed for facilitating the operation of low-power microcellular networks which will share frequency bands now being used by existing radio services. Four basic approaches have been proposed to facilitate such spectrum sharing with incumbent radio systems. A first approach, known as geographically coordinated sharing, is an adaptation of a known microwave link coordination technique, employed by terrestrial point-to-point microwave system designers. With this approach, computer models are utilized to determine inter-link interference from information concerning terrain scattering (e.g., trees and buildings), signal propagation, and antenna patterns.
A second approach, known as "geographical avoidance," operates based upon the presumption that a microwave receiver will not encounter interference from low power terminals transmitting on the same channel if those terminals utilize a level of power which is sufficiently low, and if the terminals are located far enough away from the microwave receiver sites. Significant amounts of information concerning the physical dimensions of nearby objects and complex computer-aided radio propagation modeling techniques must be utilized in order to employ this approach, and preferred propagation models used in conjunction with the geographical avoidance technique do not give an accurate picture of available microwave channels (and thus fail to provide the optimal reclamation of microwave channels which could be utilized by the mobile communications system).
A third spectrum sharing approach utilizes wideband modulation techniques, so that a low-power mobile communications system will occupy a much wider bandwidth than a higher power incumbent radio system. One example implementation of such a spectrum sharing approach utilizes a Wideband Direct Sequence Code Division Multiple Access (CDMA) spectrum overlay with a lower power level per microwave channel, in which a transmitted signal of the mobile system is spread over five microwave channels of the incumbent system, thus reducing the per-channel power by 7 dB as compared with narrow band approaches. By utilizing this technique, the size of each exclusion zone for any one microwave channel will be reduced by a factor of nearly two, assuming free-space (i.e., no shadowing) propagation. The disadvantages of this approach include the additional complexity required for each handheld terminal of the mobile communications system, and the likelihood that unwanted intersystem interference (mobile-to-fixed and fixed-to-mobile) will inevitably result in areas where there is a high density of fixed microwave systems. Moreover, there is evidence to suggest that ten PCN transmitters which employ Wideband Direct Sequence CDMA, each having a 1 mW output power and being located 2 miles from a fixed microwave site, can substantially degrade the performance of a fixed point-to-point microwave receiver. Evidence also suggests that a single PCN user, at up to 12 miles away from a fixed microwave receiver site, could potentially interfere with a fixed microwave link. See page 8 of Prabhu, V. K., "Some Considerations of Frequency Sharing Between Fixed Service and Personal Communications Network," Report to FCC Regarding GEN. Docket 90-314, Oct. 1, 1990.
In a fourth approach to spectrum sharing, adaptive notch filters are provided in mobile radio terminals which employ Wideband Direct Sequence CDMA. The notch filters each reduce or eliminate the strength of signals within a specified frequency band which are transmitted by the radio terminal. The mobile communications network controls the terminals to tune each notch filter to a particular channel, to thus prevent harmful interference to a nearby fixed microwave receiver tuned to that channel. One significant disadvantage associated with this approach is the additional costs associated with incorporating the adaptive filters in each handheld terminal, and the added complexity of coordinating the operation of the adaptive filters. In addition, the number of mobile radio terminals which can transmit at a given location is determined based on the closest microwave site of those which use any of the microwave channels over which the PCN signal is spread. Accordingly, in a dense environment of fixed microwave usage, Wideband Direct Sequence CDMA mobile communications systems will likely require larger exclusion zones than Narrowband mobile systems.